1. Field of the Invention
The present invention relates to a liquid crystal driver and a liquid crystal display device using the same and, particularly, relates to an active matrix type liquid crystal driver and a liquid crystal display device using the same.
2. Description of the Related Art
A conventional liquid crystal driver using a data driver LSI HD66310 described in Hitachi LCD driver LSI databook (Published by Hitachi Ltd., March 1994, pp. 1166-1185) will be explained below.
FIG. 2 is a configuration diagram of the conventional data driver HD66310.
In FIG. 2, the reference numeral 201 designates a data driver; 202, display data transferred from a system thereto; 203, a group of control signals for controlling the data driver; 204, a timing control circuit; 205, a control signal for controlling the timing of latching the display data 202; 206, display data; 207, a display timing signal; 208, a latch address control circuit; 209, a group of latch signals generated by the latch address control circuit 208; 210, a latch circuit for latching the display data 206 successively; 211, display data latched by the latch circuit 210 simultaneously; 212, a latch circuit for latching the display data 211 simultaneously on the basis of the timing signal 207; 213, display data latched by the latch circuit 212; 214, a level shifter for shifting a logic voltage level to a liquid crystal driving voltage level; 215, display data of voltage level shifted by the level shifter 214; 216, a reference voltage for a liquid crystal driving voltage; 217, a liquid crystal driving circuit for generating a liquid crystal driving voltage on the basis of the reference voltage 216; and 218, a group of liquid crystal driving signals for driving a liquid crystal panel.
In FIG. 2, twelve bits of display data 202, which are for four pixels (3 bits for gray scales×4 pixels), are transferred together from the system, so that display data corresponding to 160 pixels (4 pixels×40 times) are latched successively by the latch circuit 210 on the basis of the latch signal 209 generated by the latch address control circuit 208. The thus latched display data 211 corresponding to 160 pixels are further latched simultaneously by the latch circuit 212 on the basis of the timing signal 207 synchronized with a gate selection signal of a scanning driver. The voltage levels of the display data 213 are shifted to liquid crystal driving voltage levels by the level shifter 214, so that the level shifter 214 outputs display data 215. The liquid crystal driving circuit 217 selects voltage levels corresponding to the display data 215 from eight levels V7 to V0 of the reference voltage 216 and outputs the selected voltage levels as a group of liquid crystal driving signals 218. In this manner, display of eight gray scales corresponding to display data can be achieved by driving a liquid crystal panel on the basis of eight voltage levels.
FIG. 3 shows the relation between liquid crystal driving voltage and display brightness. In liquid crystal, display brightness varies correspondingly to a voltage applied to a common electrode. Therefore, display of eight gray scales is achieved by applying eight voltage levels V7 to V0 to the liquid crystal. Further, when voltages which are equal but different in polarity (positive polarity and negative polarity) are applied to the common electrode, the brightness does not change. Generally, in order to prevent the liquid crystal panel from burning, the voltage to be applied thereto is driven to alternate between positive polarity and negative polarity periodically.
FIG. 4 is a configuration diagram of a liquid crystal display device having data drivers in opposite sides of a liquid crystal panel. In FIG. 4, the reference numeral 401 designates an power supply circuit for generating reference voltages for driving liquid crystal; 402, an AC switching signal expressing AC switching timing; 403 and 404, reference voltages obtained by AC switching in different timing; 405, a scanning driver LSI (hereinafter referred to as “scanning driver”) for driving gate lines of a liquid crystal panel 411; 406, the gate lines of the liquid crystal panel 411 driven by the scanning driver 405; 407, a data driver for driving data lines arranged in the upper side of the liquid crystal panel 411; 408, the data lines driven by the data driver 407; 409, a data driver for driving data lines arranged in the lower side of the liquid crystal panel 411; 410, the data lines driven by the data driver 409; and 411, the liquid crystal panel.
FIG. 5 shows the timing of an AC switching signal which serves as a reference voltage signal for AC switching outputs in the case where data drivers are arranged in the upper and lower sides of the liquid crystal panel as shown in FIG. 4. The power supply circuit 401 generates an upper data driver reference AC voltage 403 and a lower data driver reference AC voltage 404 in synchronism with the AC switching signal 402. The upper data driver reference AC voltage 403 and the lower data driver reference AC voltage 404 are reversed to each other in the timing of polarity (positive polarity and negative polarity). The scanning driver 405 selects gate lines 406 one line by one line successively and pixels on selected one of the gate lines are driven one pixel by one pixel alternately by the upper and lower data drivers 407 and 409. Accordingly, liquid crystal cells on the gate lines successively driven by the scanning driver 405 can be driven so that liquid crystal cells on each of the gate lines alternate their polarity between positive one and negative one). As a result, the quality of an image on the display is improved.
FIG. 6 is a configuration diagram of a liquid crystal display device having a data driver in one side of a liquid crystal panel. In FIG. 6, the reference numeral 601 designates an power supply circuit for generating a reference voltage for driving liquid crystal; 602, an AC switching signal expressing AC switching timing; 603, a reference AC voltage obtained by AC switching; 604, a scanning driver for driving gate lines of a liquid crystal panel 608; 605, the gate lines of the liquid crystal panel 608 driven by the scanning driver 604; 606, a data driver for driving data lines arranged in the upper side of the liquid crystal panel 608; 607, the data lines driven by the data driver 606; and 608, the liquid crystal panel.
FIG. 7 shows the timing of an AC switching signal which serves as a reference voltage signal for AC switching an output in the case where a data driver is arranged singly in the upper side of the liquid crystal panel as shown in FIG. 6. The power supply circuit 601 generates a reference AC voltage 603 in synchronism with the AC switching signal 602. The scanning driver 604 selects gate lines 605 one by one successively so that selected one of the gate lines is driven by the upper data driver 602. Accordingly, liquid crystal cells on the gate lines successively driven by the scanning driver 604 are driven so that liquid crystal cells on one and the same gate line have the same (positive or negative) polarity. As a result, the quality of an image on the display is deteriorated.
FIG. 8 is a view showing another voltage applying method adapted to the case where the data driver shown in FIG. 6 is used. Although FIG. 7 has shown the case where the reference voltage 603 is supplied as an AC voltage, FIG. 8 shows the case where burning of the liquid crystal panel is prevented by changing both the electric potential Vcom of the common electrode (common electrode drive) and the reference voltage 603. Also in this method, all liquid crystal cells on one and the same gate line have the same (positive or negative) polarity, so that the quality of an image on the display is deteriorated.
Alternate-column inversion drive of the liquid crystal panel has an advantage in that display quality is improved with compared with the case of no use of alternate-column inversion drive, because voltages applied to liquid crystal cells are inverted on alternate columns so that the current flowing in the common electrode at the time of liquid crystal drive becomes smaller. As for the conventional data driver arrangement, therefore, data drivers are arranged in the upper and lower portions of the liquid crystal panel. On the other hand, the liquid crystal display device is on strong demands not only for high quality display but also for small size and light weight. Arrangement of one data driver in a single side makes it easy to reduce size and weight. The arrangement of one data driver in a single side of the liquid crystal panel, however, has a problem that display quality deteriorates compared with the case of alternate-column inversion drive of the liquid crystal panel.